Some mattress technologies provide somewhat uniform support using a combination of inner springs and one or more foam layers provided under a mattress cover. However, uniform support presents a problem, because the weight distribution of the human body is not at all uniform. In fact, the lumbar region is where approximately 70 percent of the body weight of a sleeper lies. Although weight distribution of a sleeper is, of course, different from person to person, for a large majority, a sleeper's hips or buttocks will tend to sink excessively far into such mattresses as the foam layers breakdown from normal use over time. This problem is exacerbated when the mattress is used by two persons sleeping together.
This deficiency in support will tend to reduce the sleeper's comfort, to a greater or lesser degree depending on the person. However, a more important effect is that this deficiency in support may permit a person to sleep in a condition of postural misalignment. Spinal alignment, in a good sleeping posture, should be the same as that in a good standing posture. Thus a sleeper should be supported so that his or her spine will be laterally straight, and will be curved with no more (and no less) than normal lumber and thoracic arch and pelvic tilt. Distortions of this sleeping posture will produce immediate or gradual discomfort, and may also lead to backaches, or to vague discomforts which reduce the sleeper's overall level of health and well-being.